test(parity): keep each route walk in order under a shuffled run

A parity file is one account walked through a route's cases, each
request reading what the earlier ones stored, and the recorded files
pin that sequence. The nightly shuffle reordered the steps and every
snapshot after the first moved. Keep each walk in order with
shuffle: false, wrapping the files whose top-level blocks build on each
other, and note the rule for order-bound files in the testing guide.

docs/contributing/testing.md

@@ -284,3 +284,14 @@ earns its place; do not extend the date.
 Order dependence is looked for on purpose: the nightly workflow runs both
 suites with `--sequence.shuffle` and prints the seed. Replay a failure with
 `--sequence.shuffle --sequence.seed=<seed>`.
+
+A case resets or states what it reads: `vi.resetAllMocks()` (or
+`mockReset()`) where a case swaps in its own implementation, since
+`clearAllMocks` keeps it and a `mockResolvedValueOnce` the code never consumed
+answers the next case; a module cache dropped through its own invalidator; a
+row the case needs written by the case or its own `beforeAll`. A file that is
+one walk by design (a parity sequence, a migration's before and after, a
+dry run followed by the import and the same file again) says so in a comment
+and passes `{ shuffle: false }` to its `describe`, which keeps that block in
+order while the rest of the suite still shuffles. Wrap the whole file in one
+such block when its top-level blocks build on each other.